6-O-methyl erythromycin A (clarithromycin) is a semi synthetic macrolide antibiotic related to erythromycin A. It exhibits excellent antibacterial activity against gram-positive bacteria, some gram-negative bacteria, anaerobic bacteria, Mycoplasma, and Chlamydia. It is stable under acidic conditions and is efficacious when administered orally. Clarithromycin is a useful therapy for infections of the upper respiratory tract in children and adults. Clarithromycin is stable under acidic conditions and is efficacious when administered orally.
The chemical structure of clarithromycin is:

Various methods of preparing 6-O-methylerythromycin A from erythromycin A have been described in the patent literature. One of the most effective methods includes the following steps: 1) protecting the 9-oxo group with a substituted oxime group, 2) protecting the hydroxyl groups in positions 2′ and 4″, 3) methylating the hydroxyl in position 6 to give a protected sililated clarithromycin oxime, and 4) removing the protecting groups at the 2′, 4″ and 9 position.
The third step, which comprises methylating the hydroxyl group at position 6, is performed in the presence of a solvent. This 6-O-methylation of various erythromycin derivatives in converting erythromycin A to clarithromycin has been reported in several U.S. Patents including U.S. Pat. Nos. 4,680,386 and 4,672,109.
U.S. Pat. No. 4,680,386 for example, describes a method of methylating the hydroxyl group at the 6 position by reacting the compound with a methylating agent in the presence of a base in an aprotic solvent at a temperature of between 0° C. and room temperature. The '386 patent describes the use of solvents including N,N-dimethylformamide, dimethyl sulfoxide, hexamethylphosphoric triamide, and a mixture of one or more of these solvents. U.S. Pat. No. 4,672,109 describes the use of solvents such as dimethyl sulfoxide, N,N-dimethylformamide, hexamethyl phosphoric triamide, a mixture of two or more of these solvents or a mixture of one of these solvents and tetrahydrofuran, 1,2-dimethoxyethane and the like. The '109 patent further describes a preferred embodiment of this step using a mixture of dimethyl sulfoxide and tetrahydrofuran. WO 97/19096 describes a mixture of solvents including N,N-dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, hexamethyl phosphoric triamide, tetrahydrofuran, 1,2-dimethoxyethane, acetonitrile and ethyl acetate for use in the methylating step.
However, several of the above-described solvents are expensive, do not enable selective methylation, produce significant unwanted side products and/or cause complications during later phase separation steps.
The fourth step includes removing the protecting groups, and thus, converts protected silylated clarithromycin oxime to clarithromycin. Described methods of converting a protected silylated clarithromycin oxime, such as S-MOP oxime, to clarithromycin include reacting the protected silylated clarithromycin oxime with ethanol in the presence of an acid and a deoximating agent. The product of the reaction is then washed with water one or more times.  The ethanol generally also contains water.
U.S. Pat. No. 4,990,602 has an ethanol to water ratio of 1:4 and does not involve cooling. U.S. Pat. No. 4,670,549 adds sodium hydroxide after cooling at an ethanol to water ratio of 1:3. Neither of these methods lowers the impurity content of clarithromycin.